1. Field of the Invention
The present invention relates to phase lock loops having programmable output frequencies, and in particular, to phase lock loops having programmable output frequencies with selectable frequency switching speeds.
2. Description of the Related Art
Phase lock loops (PLLs) have long been a fundamental element in modern communications systems, particularly those requiring frequency agility such as frequency hopping. The frequency switching, or phase locking, time of a conventional PLL is limited by the period (inverse of frequency) of the reference signal applied to one input of the phase detector. The frequency of the reference signal determines how often the PLL makes corrections to the frequency of its output signal which is fed back and compared at the reference frequency rate. Thus, if a faster frequency switching, or phase locking, time is desired, the reference frequency can be increased. Running the PLL at progressively higher reference frequencies enables the system to lock in proportionately less time.
However, the reference frequency also sets the resolution, or frequency stepping capability, of the PLL which, in turn, has an upper limit established by system level constraints, such as channel spacing or channel fine tuning. Accordingly, increasing the reference frequency beyond this limit would result in either skipping channels or fine tuning more coarsely than desired. Furthermore, increasing the reference frequency of the PLL requires an increase in the cut-off frequency of the low pass (typically) loop filter. However, this requirement of a wide loop filter bandwidth for achieving fast frequency switching times is contrary to the requirement of a narrow loop filter bandwidth for meeting the requirements for high resolution or frequency stepping capability.
Conventional PLL designs have sought to achieve both fast frequency switching times and high frequency resolution in a number of ways. For example, see E. Sharoni, "Digital Control Speeds Synthesizer Switching", Microwaves & RF, April 1987, pages 107-12, the disclosure of which is incorporated herein by reference. However, notwithstanding such improvements in conventional design techniques, further improvements would be desirable.